Exhaust emissions from internal combustion engines are a significant contributor to pollution in the environment. In particular, nitric oxide (NOx) emissions contribute to smog and acid rain. NOx, which includes both nitrogen oxide and nitrogen dioxide, is a byproduct of the combustion of fossil fuels, and diesel engines are regarded as a major generator of NOx. Diesel engines can also be a significant source of soot and other particulate matter.
To reduce the levels of soot and other particulate matters emitted into the atmosphere, the exhaust systems of diesel engines include a system for removing these materials. A Diesel Particulate filter (DPF) physically captures soot and other particulate matter in the diesel exhaust. The captured material can then be combusted once captured if the aftertreatment system is operated at a sufficiently high temperature.
To reduce the levels of NOx emitted into the atmosphere, the exhaust systems of diesel engines include a system for the Selective Catalytic Reduction (SCR) of NOx in which a urea solution is injected in the exhaust stream upstream of the catalytic converter. After injection into the exhaust stream, the urea solution evaporates and mixes with the exhaust stream. The urea decomposes while in the exhaust system and hydrolyzes into ammonia. NOx then reacts with the thus generated ammonia in the presence of the catalyst and is catalytically reduced to non-polluting nitrogen, water and carbon dioxide.
There is an ongoing desire to further reduce exhaust emissions. Accordingly, there is an ongoing need for improvements in exhaust treatment systems.